Various thermal transfer recording methods have hitherto been known in the art. Among others, a method is proposed to form various full color images by utilizing sublimation dye as a recording material, and thermally transferring the sublimation dye from a thermal transfer sheet onto a transfer-receiving material which can be dyed with sublimation dye, wherein the thermal transfer sheet comprises a dye layer formed by holding the sublimation dye by a suitable binder on a substrate such as a polyester film, and wherein the transfer-receiving material includes the thermal transfer image-receiving sheet comprising a dye receiving layer provided on paper, plastic film or the like. In this case, a large number of color dots of three or four colors with the quantity of heat being regulated are transferred by heating by means of a thermal head as heating means in a printer onto the receiving layer in the thermal transfer image-receiving sheet to reproduce a full color of an original by the multicolor dots. In this method, since coloring materials used are dyes, the formed images are very sharp and are highly transparent and thus are excellent in reproduction of intermediate colors and in gradation and are comparable with images formed by conventional offset printing or gravure printing. At the same time, this method can form high-quality images comparable with full-color images formed by photography.
In the thermal transfer recording method utilizing the sublimation transfer, an increase in printing speed of thermal transfer printers has posed a problem that conventional thermal transfer sheets cannot provide satisfactory print density. Further, high density and high sharpness have become required of prints of images formed by thermal transfer. Therefore, image quality becomes lowered at a black high density area when a black thermal transfer image is formed by superimposing sequentially three color dyes including each dye layer of yellow, magenta and cyan, and a protective layer which is finally transferred from a protective layer transfer sheet. More specifically, a phenomenon, so-called “burnt deposit” comes to appear, since the receiving layer of the thermal transfer image-receiving sheet fuses to the dye layer of the thermal transfer sheet. In this context, the “burnt deposit” is a phenomenon that a change of color phase occurs in a black part and thereby the surface of the printed matter becomes matte and lose luster. For this reason, many attempts have been made in order to improve a thermal transfer sheet or a thermal transfer image-receiving sheet onto which an image is formed by receiving a sublimation dye which is transferred from the thermal transfer sheet. For example, an attempt has been made in order to improve transfer sensitivity in printing by reducing a thickness of the thermal transfer sheet. However, this method has a problem that wrinkles are caused or even breakage is occurred, due to heat, pressure or the like, during production of the thermal transfer sheet or during thermal transfer recording.
Another attempt has been made in order to improve transfer sensitivity in printing or improve a print density, by increasing a ratio of dye to binder resin (dye/binder) in the dye layer of a thermal transfer sheet. In this method, however, during storage in a wound state, the dye is transferred onto the heat-resistant slip layer provided on the backside of the thermal transfer sheet, and, at the time of rewinding, the dyes transferred onto the heat-resistant slip layer are retransferred (kicked back) onto dye layers of other colors or the like. If the contaminated dye layer is thermally transferred onto a thermal image-receiving sheet, the color is deviated from the designated color or a phenomenon, so-called “scumming” is caused. Further, there have been attempts that high energy is applied on a thermal transfer printer, different from a thermal transfer sheet, during thermal transferring to form an image. In this method, however, a dye layer is likely to fuse to the image-receiving layer, resulting in an abnormal transfer. If a great amount of releasing agent is added to the image-receiving layer in order to prevent the abnormal transfer, image blurring or scumming arises.
For example, Patent Document 1 proposes a thermal transfer sheet provided with: a substrate; an adhesive layer formed on the substrate and containing a water soluble curing agent and a water soluble resin having activated hydrogen; and a dye layer formed on the adhesive layer. Patent Document 2 discloses a thermal transfer sheet wherein a hydrophilic barrier/subbing layer comprising polyvinylpyrrolidone as a main component and, mixed with the main component, polyvinyl alcohol as a component for enhancing dye transfer efficiency is provided between a dye layer and a support. Furthermore, Patent Document 3 discloses a thermal transfer sheet provided with: a base film; and a recording layer containing sublimation dye, between of which an intermediate layer is provided. This intermediate layer contains another sublimation dye having a diffusion coefficient smaller than that of the sublimation dye contained in the recording layer. In this reference, it is only mentioned that hydroxyethyl cellulose is used as the intermediate layer.
In the thermal transfer sheet of Patent Document 1, the curing agent needs to be added to the adhesive layer to cure the water soluble resin. And, it is required to control an amount of the curing agent to be added. If the curing reaction is insufficient, more amount of dye transfers to the adhesive layer, and less amount of dye transfers from the dye layer to the image-receiving sheet. As a result, a high density print cannot be obtained. Furthermore, since the cohesion of the adhesive layer (undercoat layer) is insufficient, an abnormal transfer occurs in the print after being stored at high temperature and high humidity. On the other hand, in thermal transfer sheets of Patent Documents 2 and 3, the printed matter obtained by using these transfer sheets does not achieve the sufficient level of the maximum density.
Other prior art documents, Patent Documents 4 and 5 disclose that an intermediate layer containing a metal or a metal oxide is provided between a substrate and a dye layer in a thermal transfer sheet. Patent Document 4 discloses, in its Example, that a dye is transferred onto an activated clay paper by using a thermal transfer sheet obtained by depositing a metal or a metal oxide on a substrate and depositing a thin layer of dye thereon. However, thus obtained thermal transfer sheet cannot provide the sufficient sharpness and the sufficiently high density of the thermal transfer image. Furthermore, the production cost is high, since a special apparatus is required for depositing.
In Patent Document 5, an easily adhesive layer is provided between a thermal transfer sheet substrate and a dye layer. The easily adhesive layer contains a homopolymer of N-vinylpyrrolidone, or a copolymer of N-vinylpyrrolidone with other components. Furthermore, in order to improve the adhesiveness, an inorganic filler such as ultraviolet (UV) absorber, or other fillers such as silica or alumina is added to the easily adhesive layer. However, the easily adhesive layer shows low transfer sensitivity and cannot provide the high density print, although the adhesiveness with the substrate of the dye layer can be improved.
In order to improve the transfer sensitivity in printing, for example Patent Document 6 proposes a thermal transfer sheet provided with an adhesive layer containing polyvinylpyrrolidone resin and modified polyvinylpyrrolidone resin between a substrate and a dye layer. However, this thermal transfer sheet cannot provide a sufficient level of the print density, although the abnormal transfer can be prevented.
Furthermore, Patent Document 7 discloses a thermal transfer sheet provided with an adhesive layer between a substrate and a dye layer, in which the adhesive layer contains a thermoplastic resin which is a polyvinylpyrrolidone resin or polyvinylalcohol resin; and colloidal inorganic pigment ultrafine particles. However, this thermal transfer sheet may cause the abnormal transfer, in printing after being stored at high temperature and high humidity.
As mentioned above, there have not been found a thermal transfer sheet which can provide a high transfer sensitivity in printing, and a high density print, and can prevent an abnormal transfer in printing even after being stored at high temperature and high humidity.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-262594
Patent Document 2: Japanese Patent examined-Application Publication No. H7-102746
Patent Document 3: Japanese Patent examined-Application Publication No. H5-69718
Patent Document 4: Japanese Patent Application Laid-open No. S59-78897
Patent Document 5: Japanese Patent Application Laid-open No. 2003-312151
Patent Document 6: Japanese Patent Application Laid-open No. 2005-231354
Patent Document 7: Japanese Patent Application Laid-open No. 2006-150956